Position detecting method with observation of position detecting marks

ABSTRACT

A wafer with an exposure surface and an exposure mask are disposed, directing the exposure surface to the exposure mask with a gap being interposed therebetween, the wafer having a position aligning wafer mark formed on the exposure surface, the wafer mark having a linear or point scattering source for scattering incident light, and the exposure mask having a position aligning mask mark having a linear or point scattering source for scattering incident light. A relative position of the wafer and exposure mask is detected by applying illumination light to the wafer mark and mask mark and observing scattered light from the scattered sources of the wafer mark and mask mark.

This is a division of application Ser. No. 08/640,170 filed Apr. 30,1996, now U.S. Pat. No. 5,827,629.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a method of detecting a relativeposition of a mask and a wafer, and to a position detecting mark (analignment mark). More particularly, the invention relates to a positiondetecting method (an alignment method) suitable for improving throughputof proximity exposure, and to a position detecting mark.

b) Description of the Related Art

A vertical detection method and an oblique detection method are known asa method of detecting the positions of a wafer and a mask by using analigner having a lens system combined with an image processing system.The vertical detection method observes an alignment mark along adirection perpendicular to the mask plane, and the oblique detectionmethod observes it obliquely.

A chromatic bifocal method is known as a focussing method used by thevertical detection method. The chromatic bifocal method observesalignment marks formed on a mask and a wafer by using light of differentwavelengths and chromatic aberrations of the lens system, and focusesthe images of the masks on the same flat plane. An absolute precision ofposition detection by the chromatic bifocal method can be made highbecause the optical resolution of the lens system can be set high inprinciple.

However, since an alignment mark is observed vertically, a part of theoptical system enters the exposure area. Since the optical systemshields exposure light, it is necessary to retract the optical systemfrom the exposure area when exposure light is applied. A time requiredfor retracting the optical system lowers throughput. The alignment markcannot be observed during the exposure, which is one of the reasons oflowering an alignment precision during the exposure.

With the oblique detection method, the optical axis of the opticalsystem is disposed obliquely to the mask plane, and the system can bedisposed without shielding the exposure system. It is thereforeunnecessary to extract the optical system during exposure, permittingobservation of an alignment mark even during the exposure. Therefore,throughput does not lower and position misalignment during the exposurecan be prevented.

A conventional oblique detection method uses oblique focussing in whichregular reflection light reflected from the mark is obliquely focussedto detect the image of the mark. An absolute precision of positiondetection is therefore lowered by image distortion. Furthermore, sinceregular reflection light is incident to an observation lens, the opticalaxis of illumination light cannot coincide with the optical axis ofobservation light. Since the optical axes of illumination andobservation light are required to be separated, if there is even aslight shift between both the axes, the detection precision is loweredand the installation of the optical system becomes complicated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alignment methodcapable of detecting a position with high alignment precision evenduring exposure and without lowering throughput.

It is another object of the present invention to provide a semiconductorsubstrate and an exposure mask having alignment marks, capable ofdetecting a position with high alignment precision even during exposureand without lowering throughput.

According to one aspect of the present invention, there is provided amethod of detecting a position, comprising the steps of: disposing awafer with an exposure surface and an exposure mask, directing theexposure surface to the exposure mask with a gap being interposedtherebetween, the wafer having a position aligning wafer mark formed onthe exposure surface, the wafer mark having a linear or point scatteringsource for scattering incident light, and the exposure mask having aposition aligning mask mark having a linear or point scattering sourcefor scattering incident light; and detecting a relative position of thewafer and the exposure mask by applying illumination light to the wafermark and the mask mark and observing scattered light from the scatteredsources of the wafer mark and the mask mark.

Generally, if the illumination and observation axes are made coaxial andthe axes are set obliquely to the exposure plane, regular reflectionlight from the wafer mark and mask mark does not return along theobservation optical axis. Therefore, the images of these marks cannot beobserved. Regular reflection means a reflection that if parallel lightfluxes are applied, reflected light is also parallel and that theincident angle and reflection angle are the same. If scattering sourcesfor scattering incident light are formed on the wafer mark and maskmark, light fluxes among scattered light fluxes in the aperture of anobject lens of the observation optical system form an image so thatscattered light can be observed.

According to another aspect of the present invention, there is provideda semiconductor substrate comprising: an exposure plane formed with aposition aligning wafer mark having a plurality of edge type or pointtype scattering sources for scattering incident light, the scatteringsources being disposed in the direction perpendicular to the plane ofincidence of the incident light.

According to a further aspect of the present invention, there isprovided an exposure mask comprising: a position aligning mask markhaving a plurality of edge type or point type scattering sources forscattering incident light, the scattering sources being disposed in thedirection perpendicular to the plane of incidence of the incident light.

If a plurality of scattering sources of the wafer mark and mask mark aredisposed in a direction perpendicular to the incidence plane, images ofscattered light from a plurality of scattering sources can be formed atthe same time. If a position is detected by observing at the same timethe images of scattered light from the plurality of scattering sources,a position detection error to be caused by a variation of the shapes ofeach scattering source at the manufacture processes can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wafer illustrating reflection from awafer mark, and edge scattering light.

FIG. 2A is a schematic cross sectional view of a position detectionapparatus used by embodiments of the invention.

FIG. 2B is a plan view of wafer marks and a mask mark.

FIG. 2C is a diagram showing images formed by edge scattering light fromwafer and mask marks, and a light intensity distribution in image-plane.

FIG. 2D is a cross sectional view showing the wafer and mask surfacesnear the object surface.

FIG. 3A is a plan view of a wafer mark used for observation experimentsof edge scattering light.

FIGS. 3B and 3C are cross sectional views of the wafer mark used forobservation experiments of edge scattering light.

FIG. 4A is a diagram sketched from a photograph of an edge scatteringlight image of the wafer mark shown in FIG. 3B.

FIG. 4B is a diagram sketched from a photograph of a vertically detectedimage of the wafer mark shown in FIG. 3B.

FIG. 5 is a diagram showing an image signal of an edge scattering lightimage of the wafer mark shown in FIG. 3B.

FIGS. 6A and 6B are graphs showing the results of displacementmeasurements through image signal processing.

FIG. 7A is a plan view of a wafer mark or mask mark according to asecond embodiment.

FIGS. 7B to 7D are cross sectional views of wafers with wafer marks.

FIG. 7E is a cross sectional view of a mask with a mask mark.

FIG. 7F is a diagram sketched from a metal microscopy photograph of edgescattering light from the wafer mark shown in FIG. 7A.

FIG. 7G is a diagram sketched from an image of edge scattering lightfrom the wafer mark shown in FIG. 7A, the image having been taken by atelevision camera.

FIGS. 8A to 8F are diagrams showing signal waveforms at each scan lineof the image taken by the television camera as shown in FIG. 7G.

FIG. 9A is a plan view of alignment marks according to a thirdembodiment.

FIG. 9B is a cross sectional view taken along one-dot-chain line B6--B6in FIG. 9A.

FIG. 9C is a cross sectional view taken along one-dot-chain line C6--C6in FIG. 9A.

FIG. 9D1-9D3 are diagrams showing image signals of images of edgescattering light.

FIG. 9E is a graph showing a correlation function of the image signalshown in FIG. 9D3.

FIG. 10A is a cross sectional view of alignment marks according to afourth embodiment.

FIG. 10B is a diagram showing an image signal of an image of edgescattering light.

FIG. 10C is a graph showing a correlation function of the image signalshown in FIG. 10B.

FIGS. 11A, 11C, and 11E are perspective views of one edge patternconstituting a wafer mark.

FIGS. 11B and 11D are diagrams showing images by edge scattering lightfrom the edges shown in FIGS. 11A and 11C.

FIG. 12 is a graph showing a signal of a point image formed byscattering light from an apex.

FIGS. 13A to 13C are plan views of wafer and mask marks having an apexfrom which illumination light is scattered.

FIG. 14A is a cross sectional view of alignment marks according to afifth embodiment.

FIG. 14B is a diagram showing a signal of an image formed by edgescattering light.

FIG. 15A is a plan view of a wafer mark according to a sixth embodiment.

FIG. 15B is a schematic cross sectional view of the wafer mark shown inFIG. 15A and an observation optical system in which the wafer marks areobserved obliquely.

FIG. 15C is a graph illustrating a detected position dependency uponwafer positions in which the wafer mark shown in FIG. 15A are observedby the method illustrated in FIG. 15B to detect the positions ofrespective edges.

FIG. 16 is a cross sectional view of wafer marks and a mask markaccording a sixth embodiment.

FIGS. 17A to 17C are plan views illustrating the disposal of opticalsystems relative to an exposure area.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, edge scattering light to be observed by the embodiments of theinvention will be described with reference to FIG. 1.

FIG. 1 is a perspective view of a position alignment wafer mark formedon the wafer surface 1. A projection 2 of a rectangular surface shape isformed on the wafer surface 1. Consider the coordinate system having anx-axis and a y-axis parallel to respective sides of the rectangle. Asillumination light having an incidence plane perpendicular to the y-axisis obliquely applied to the wafer surface 1, light 3 incident to themirror area of the projection 2 such as a top flat surface is regularlyreflected, whereas light 4 incident to the edge portion is scattered. Inthis specification, a regular reflection means a reflection that asparallel light fluxes are applied, reflected light fluxes are alsoparallel and that the incidence angle is equal to the reflection angle.

Consider now that the wafer surface 1 is observed with an optical systemhaving an object lens 5 whose optical axis is coaxial with the incidenceoptical axis. Light regularly reflected from the upper flat surface orthe like of the projection 2 is not incident to the object lens 5 and animage of the wafer mark is not focussed by regular reflection light. Incontrast, light scattered from an edge is radiated omnidirectionally,and part of the scattered light beams is incident to the object lens.Therefore, this scattered light can be observed in the same direction asthe incidence direction of illumination light. In one type ofembodiments of the invention, the position of a wafer is detectedthrough observation of light scattered from an edge.

FIG. 2A is a schematic cross sectional view of a position detectionapparatus used by the embodiments of the invention. The positiondetection apparatus is structured by a wafer/mask holding unit 10, anoptical system 20, and a controller 30.

The wafer/mask holding unit 10 is constituted by a wafer holder 15, amask holder 16, and a driver mechanism 17. For position alignment, awafer 11 is held on the top surface of the wafer holder 15, and a mask12 is held on the bottom surface of the mask holder 16. The wafer 11 andmask 12 are disposed in parallel, forming a constant gap between theexposure surface of the wafer 11 and the bottom surface (mask surface)of the mask 12. Position alignment wafer marks 13 are formed on theexposure surface of the wafer 11, and a position alignment mask mark 14is formed on the mask surface of the mask 12. The wafer marks and maskmark are collectively called alignment marks hereinafter.

The wafer marks 13 and mask mark 14 have edges from which incident lightis scattered. If light beams incident to these marks, the light incidentto the edge is scattered and the light incident to other areas isregularly reflected.

The drive mechanism 17 can generate a relative motion between the waferholder 15 and mask holder 16. Taking an X-axis along the direction fromthe left to right in FIG. 2A, a Y-axis along the direction perpendicularto the drawing sheet from the front side to back side thereof, and aZ-axis along the direction normal to the exposure surface, then arelative motion between the wafer 11 and mask 12 can be realized in theX-, Y-, and Z-directions and in the rotation direction (θ_(Z) direction)about the Z-axis. A relative motion can be realized also in the rotationdirections (θ_(X) and θ_(Y) directions) about the X- and Y-axes.

The optical system 20 is structured by an image detector 21, a lens 22,a half mirror 23, and a light source 24.

The optical axis 25 of the optical system 20 is obliquely set relativeto the exposure surface. Illumination light radiated from the lightsource 24 is reflected by the half mirror 23, propagates as light fluxesalong the optical axis 25, and are obliquely applied via the lens 22 tothe exposure surface. The light source 24 is disposed at the focal pointon the image side of the lens 22 so that the illumination light radiatedfrom the light source 24 is collimated by the lens 22 and transformedinto parallel light fluxes. The light source 24 is adapted to adjust theintensity of illumination light.

Of light scattered at the edges of the wafer marks 13 and mask mark 14,light incident to the lens 22 is converged by the lens 22 and focussedon the light reception surface of the image detector 21. In this opticalsystem 20, therefore, illumination is telecentric illumination and theillumination and observation optical axes are the same optical axis.

The image detector 21 photoelectrically converts the images of wafer andmask marks focussed on the light reception plane into image signals. Theimage signals are inputted to the controller 30.

The controller 30 processes image signals supplied from the imagedetector 21 to detect a relative position of the wafer marks 13 and maskmark 14 in the direction of Y axis. The controller 30 sends controlsignals to the driver mechanism 17 so as to make the wafer marks 13 andmask mark 14 have a predetermined relative position. In response to thiscontrol signal, the drive mechanism 17 moves the wafer holder 15 or maskholder 16.

FIG. 2B is a plan view showing the relationship of a relative positionbetween the wafer marks 13 and mask mark 14. Three rectangular patternseach having four sides in parallel with the X- or Y-axis are disposed inthe X-axis direction, forming one mark. One mark may be constituted byfour or more rectangular patterns as will be later described. The maskmark 14 is interposed between a pair of wafer marks 13.

The wafer marks 13 and mask mark 14 have the cross section shown in FIG.2A taken along one-dot-chain line A2--A2 in FIG. 2B. Illumination lightincident to the wafer marks 13 and mask mark 14 is scattered at edges ofthe rectangular patterns of FIG. 2B facing the optical axis. Lightradiated to the areas other than the edges is regularly reflected anddoes not enter the lens 22. Therefore, the image detector 21 can detectonly the light scattered by the edges and entered the lens 22.

Next, the nature of an image formed by edge scattered light will bedescribed.

The light intensity distribution I of an image formed by incoherentmonochrome light is given by:

    I(x, y)=∫∫O(x-x', y-y')PSF(x', y')dx'dy'         (1)

where O(x, y) represents an intensity distribution of light reflectedfrom the surface of an observation object, PSF(x, y) represents a pointspread function of the lens, and the integration is performed for thewhole surface of the observation object.

Each edge of the rectangular pattern shown in FIG. 2B can be consideredas a series of fine points disposed in parallel with the Y-axis fromwhich light is reflected. A reflected light intensity distribution ofeach fine point is assumed to be the Dirack delta function δ. Theintensity distribution of light scattered from a fine point can beapproximated to the delta function in practice. Assuming that the edgeextends in the Y-axis direction within the range satisfying theisoplanatism of the lens, then O(x, y)=δ(x) The equation (1) can betransformed into: ##EQU1## wherein I(x) is a line spread function of thelens and can be written by:

    I(x)=LSF(x)                                                (3)

where LSF(x) represents a line spread function of the lens.

If illumination light has continuous spectra, I(x) is given by:

    I(x)=∫LSFλ(x-Δxλ)dλ        (4)

where λ represents a wavelength of light, LSFλ represents a line spreadfunction at the wavelength λ, Δxλ represents a lateral shift amount of aline image caused by the lens chromatic aberration at the wavelength λ,and the integration is performed for the whole wavelength range.

It can be understood from the equation (4) that observing lightscattered from an edge is equivalent to observing the line spreadfunction of the lens. Therefore, a stable image can be obtained throughobservation of light scattered from an edge, without being affected bythe in-plane intensity distribution of light reflected from theobservation object.

The left side of FIG. 2C shows the images focussed on the lightreception plane of the image detector 21 of FIG. 2A. Taking thedirection of intersecting the incidence plane including the observationoptical axis with the light reception plane as the x-axis and thedirection perpendicular to the x-axis in the light reception plane asthe y-axis, an image of one edge becomes a straight line shape inparallel to the y-axis. Therefore, the image of each mark has threestraight line shapes in parallel to the y-axis and disposed in thex-axis direction.

Between a pair of images 13A formed by light scattered at the edges ofthe wafer marks 13, an image 14A is formed by light scattered at theedges of the mask mark 14. Since the observation optical axis is obliqueto the exposure plane, the wafer mark images 13A and mask mark image 14Aare detected at different positions along the x-axis direction. Theright side of FIG. 2C shows an intensity distribution of images of wafermarks 13A and mask mark 14A along the y-axis direction. The distancebetween the center of one of the wafer mark images 13A and the center ofthe mask mark image 14A in the y-axis direction is represented by y1,and the distance between the center of the other of the wafer markimages 13A and the center of the mask mark image 14A in the y-axisdirection is represented by y2. Through the measurements of thedistances y1 and y2, the relationship of a relative position in they-axis between the wafer marks 13 and mask mark 14 can be known.

For example, if the mask mark is to be centered between a pair of wafermarks in the y-axis direction, one of the wafer and mask is movedrelative to the other to make y1 be equal to y2. In this manner,position alignment in the Y-axis illustrated in FIG. 2B can be achieved.By preparing three sets of position alignment marks and the opticalsystem shown in FIGS. 2A and 2B, position alignment in the X- and Y-axisdirections and in the θ direction can be achieved. In FIG. 2A, theoptical axes of illumination and observation light are made coaxial. Thecoaxial arrangement is not necessarily required, but other arrangementsare possible if regular reflection light does not enter the object lensof the observation optical system and only scattered light enters theobject lens.

Next, a method of measuring a gap between the exposure plane and maskplane will be described. An object point focussed on the light receptionplane of the image detector 21 is on the plane perpendicular to theoptical axis in the object space of the optical system 20. This plane iscalled hereinafter an "object surface".

Of the edges of wafer and mask marks, the edge on the object surfacebecomes in focus on the light reception plane. However, the edge not onthe object surface becomes more out of focus as it moves apart from theobject surface. Therefore, of the edges of each mark, the image on theobject surface becomes clearest, and the image becomes more out of focusat the position further away in the x-axis direction. In this case, theimage of the edge on the object surface is not an obliquely focussedimage but a vertically focussed image.

In FIG. 2C, a distance x1 corresponds to a distance in the x-axisdirection between the wafer mark image 13A on the object surface and themask mark image 14A on the object surface. This distance x1 is generallyequal to the distance between the points which are obtained byvirtically projecting the images on the image surface corresponding tothe wafer mark and the mask mark on the object surface onto the plane ofincidence.

FIG. 2D is a cross sectional view showing the incidence planes of thewafer surface 11 and mask surface 12 near the object surface. A point Q₂is on a line intersecting the wafer surface 11 with the object surface,and a point Q₁ is on a line intersecting the mask surface 12 with theobject surface. The line segment Q₁ -Q₂ is equal to x1/N where N is afocussing magnification.

Representing the length of the line segment Q₁ -Q₂ by L(Q₁ Q₂), the gapδ between the wafer surface 11 and mask surface 12 is given by:

    δ=L(Q.sub.1 Q.sub.2)×sin(α)              (5)

where α represents an angle between the normal direction of the wafersurface 11 and the optical axis 25. Therefore, the gap δ can be known byobtaining the length of the line segment Q₁ -Q₂ from the measureddistance x1 in FIG. 2C. In order to precisely know the gap δ, it ispreferable to precisely measure the distance x1. To this end, the depthof focus of the lens is preferably shallow.

A proximity gap between the wafer surface 11 and mask surface 12 can beset to a desired value, by controlling the driver mechanism 17 in theZ-axis so as to make the measured value x1 become a target value of thedistance x1 preset to the controller 30.

Next, a first embodiment will be described in which light scattered froma wafer mark is observed.

FIG. 3A is a plan view of a wafer mark used by the first embodiment. Awafer mark is constituted by three rectangular patterns disposed inparallel. The width of the rectangular pattern is 6 μm and the lengththereof is 100 μm. Each rectangular pattern is made of a step formed onthe surface of a wafer, and has edges from which incident light isscattered. In the following, such a rectangular pattern having edgeswhich scatter incident light, is called an edge pattern.

FIGS. 3B and 3C are cross sectional views taken along one-dot-chain lineB3--B3 of FIG. 3A. In the case of the wafer shown in FIG. 3B, a resistpattern 41 (Microposit 2400 manufactured by Shipley Co.) is formed onthe surface of a silicon substrate 40. The height H1 of the resistpattern 41 is 1.2 μm and the width W thereof is 6 μm.

The distances between the center line of the middle edge pattern and thecenter lines of edge patterns on both sides of the middle edge patternare represented by y3 and y4. Ten types of wafer marks with y3-y4 beingset to 0 nm, 20 nm, 40 nm, 60 nm, . . . , 180 nm were formed on thewafer used. The value y3-y4 is called hereinafter a shift amount of themiddle edge pattern. Each mark has a value y3+y4 of 26 μm.

In the case of the wafer shown in FIG. 3C, silicon projections 44 areformed on a silicon substrate 40. The height H2 of the projection 44 is0.5 μm. Covering the surface of the silicon substrate 40, aphosphosilicate glass (PSG) film 42 of 0.7 μm thick and a resist film 43of 1.45 μm thick are stacked in this order. The width and interval ofthe projections 44 are the same as those of the resist patterns 41 shownin FIG. 3B.

FIG. 4A shows images of wafer marks formed by the resist patterns shownin FIG. 3B, the image being obliquely observed as illustrated in FIG.2A. A microscope used for the observation has an object lens with anumerical aperture NA of 0.4, and a detection magnification of 100. Anoptical system (coaxial observation/illumination optical system) havingthe observation optical axis coaxial with the illumination optical axiswas used for the observation, in which the incidence plane of theillumination optical axis is in parallel to the longitudinal direction(X direction) of each edge pattern shown in FIG. 3A and the anglebetween the incidence plane and the normal to the wafer surface is 30degrees. In the example shown in FIG. 4A, images of three wafer marksare observed. Each wafer mark has three juxtaposed line images whichcorrespond to the images formed by light scattered from shorter sides ofthe edge patterns shown in FIG. 3A.

Images under the three juxtaposed line images (corresponding to the linespread function of the lens) are formed by edge scattering light fromserial number marks formed under the wafer marks. The influence by theserial number mark images can be alleviated if the image detector scanslaterally as viewed in FIG. 4A and detects the image signals only fromthe scan lines riding over the three juxtaposed line images.

FIG. 4B shows images of the wafer marks of resist patterns shown in FIG.3B observed by a usual microscope along the normal direction of theexposure plane. In FIG. 4B, images for the three wafer marks are shown.The edge scattering light observed in FIG. 4A is generated at theshorter side edges of each wafer mark shown in FIG. 4B. The numericalmark under each mark is a serial number of the wafer mark.

FIG. 5 shows an image signal of the line images formed by lightscattered from the edges of the middle wafer mark shown in FIG. 4A. Theabscissa represents a position on the wafer surface along the Ydirection in FIG. 3A, and the ordinate represents a light intensity.Three sharp rectangular signals (peaks) appear in correspondence withthe three line images. In this manner, an image signal representative ofthe rectangular signals (peaks) corresponding to the edge can beobtained by detecting the edge scattering light.

In FIGS. 4A, 4B, and 5, the images and image signal obtained byobserving the wafer marks of resist patterns of FIG. 3B are shown.Similarly, also in the case of the wafer marks having a laminatestructure shown in FIG. 3C, sharp images and a sharp image signal with ahigh S/N ratio were obtained.

FIGS. 6A and 6B show the measurement results of the shift amounts y3-y4of the middle edge patterns obtained through image signal processing.FIG. 6A uses the wafer marks of resist pattern shown in FIG. 3B, andFIG. 6B uses the wafer marks of the laminate structure shown in FIG. 3C.The abscissa represents a serial number of wafer marks. The shift amounty3-y4 of the wafer mark with the serial number n is n×20 nm. Theordinate represents the shift amount y3-y4 in the unit of nm obtainedthrough experiments.

In FIGS. 6A and 6B, a symbol represents a shift amount obtained throughvertical detection with coaxial observation/illumination, and a symbolrepresents a shift amount obtained through oblique detection with edgescattered light. The shift amount observed by edge scattering light wascalculated by similar pattern matching (Japanese Patent Laid-openPublication No. 2-91502, from 14-th line in the lower left column atpage 4 to the 3-rd line in the upper left column at page 7).

A method of measuring a shift amount by similar pattern matching will bebriefly described. First, a differential image signal emphasizing thecontrast of the image signal shown in FIG. 5 is obtained. Thedifferential waveform of the middle rectangular signal (peak) is movedleft to superpose it upon the differential waveform of the leftrectangular signal (peak), and the shift amount with a maximumcorrelation value is set to the distance y3. Similarly, the differentialwaveform of the middle rectangular signal (peak) is moved right tosuperpose it upon the differential waveform of the right rectangularsignal (peak), and the shift amount with a maximum correlation value isset to the distance y4. From the obtained distances y3 and y4, the shiftamount y3-y4 is calculated.

In order to more precisely measure the distances y3 and y4 and increasethe correlation value, it is preferable to make each rectangular signal(peak) waveform analogous.

As shown in FIG. 6A, in the case of the wafer marks of resist patterns,the shift amounts y3-y4 obtained by detecting edge scattering light aregenerally equal to those obtained through vertical detection, for thewafer marks of all the serial numbers 0 to 9.

As shown in FIG. 6B, in the case of the wafer marks of siliconprojections, the shift amounts y3-y4 obtained by detecting edgescattering light are slightly larger than those obtained throughvertical detection, for the wafer marks of all the serial numbers 0 to9. An increment of the shift amount observed was about 13 nm. As will bedescribed later with embodiments to follow, this increment may beexpected to be made small by forming a wafer mark with a plurality ofedge patterns, or by other measures.

Next, a second embodiment will be described wherein light scattered fromwafer and mask marks with alignment precision evaluating vernierpatterns is observed.

FIG. 7A is a plan view of a wafer mark and a mask mark to be observed.Rectangular patterns laterally long as viewed in FIG. 7A are disposed inthe vertical direction at a pitch of 4 μm.

Observations were performed for wafers having wafer marks made ofresist, polysilicon, or aluminum such as shown in FIG. 7A and maskhaving mask mark as shown in FIG. 7A. FIGS. 7B to 7E are partial crosssectional views of the wafers and mask taken along one-dot-chain lineB7--B7 of FIG. 7A.

FIG. 7B shows a wafer with wafer marks made of resist. On the surface ofa silicon substrate 40, resist patterns 41 are formed. The height of theresist pattern is 1.8 μm.

FIG. 7C shows a wafer with wafer marks made of polysilicon. An SiO₂ film51 is formed on the surface of a silicon substrate 50. A wafer mark 52of polysilicon is formed on the surface of the SiO₂ film 51. Coveringthe surface of the SiO₂ film 51 and the wafer mark 52, a resist film 53is coated. The thickness of the SiO₂ film 51 is 102.6 nm, the thicknessof the wafer mark is 198.6 nm, and the thickness of the resist film 53is 1.8 μm. Assuming that this substrate is formed by MOSFET manufactureprocesses, the SiO₂ film 51 corresponds to the gate insulating film, andthe polysilicon wafer mark 52 corresponds to the gate electrode.

FIG. 7D shows a wafer with wafer marks made of aluminum. An SiO₂ film 61is formed on the surface of a silicon substrate 60. A wafer mark 62 ofaluminum is formed on the surface of the SiO₂ film 61. Covering thesurface of the SiO₂ film 61 and the wafer mark 62, a resist film 63 iscoated. The thickness of the wafer mark is 523 nm, and the thickness ofthe resist film 63 is 1.8 μm. On the surface of the wafer mark 62, athin silicon film is formed as an antireflection film.

FIG. 7E shows a mask with a mask mark. A mask mark 71 is formed on thebottom surface of an X-ray transmission film (membrane) 70 made of SiN,the mask mark being made of tantalum functioning as an X-ray absorptionmember. The thickness of the X-ray transmission film 70 is 2 μm, and theheight of the mask mark 71 is 0.75 μm.

Edge scattering light from the samples shown in FIGS. 7B to 7E wasobserved along the direction (direction indicated by an arrow 42 in FIG.7B) slanted downward by 30 degrees from the normal direction of thewafer surface (drawing surface) of FIG. 7A. Scattered light from thewafer marks of FIGS. 7B to 7D was detected through the membrane of theX-ray mask. A metal microscope used for the observation has an objectlens with a numerical aperture NA of 0.4 and a detection magnificationof 100. Illumination light is white light radiated from a halogen lamp,and illumination is coaxial observation/illumination with telecentricillumination.

FIG. 7F is a diagram sketched from a photograph of focussed lightscattered from the edges of wafer (FIG. 7C) with wafer marks made ofpolysilicon. At the area about one third from the top of the image shownin FIG. 7F, two images of edge scattering light in the plane of theobject surface are focussed clearly. Images of light scattered from theupper and lower edges are unsharp because the edge is spaced apart fromthe object surface. As above, the light scattered from the edge in theplane of the object surface can be focussed clearly, being equivalent tothe vertical detection, and image distortions as with the obliquedetection are not generated. Since white light is used as theillumination light, light interference between the mask and wafer wasnot observed. Clear images were detected from the masks or wafers shownin FIGS. 7B, 7D, and 7E, similar to the wafers shown in FIG. 7C.

FIG. 7G shows images taken by a television camera with an opticalmagnification of 100 and an electrical magnification of 9.3. Thehorizontal scan direction is in the lateral direction (Y direction) ofFIG. 7A. The pitch between scan lines is 15 μm as converted to thedistance on the object surface. That is, the pitch between scan linesonly in the even field is 30 nm. As shown in FIG. 7G, three images atthe central area are clearly focussed, and the upper and lower imagesare unsharp. Clear images were detected also for other samples, similarto the wafers shown in FIG. 7C.

FIGS. 8A to 8F show signal waveforms corresponding to scan lines takinga clear upper third image. FIGS. 8A to 8F are signal waveformscorresponding to the 120-th to 125-th scan lines in the even field. Theabscissa represents the lateral direction (Y direction) of FIG. 7A andthe ordinate represents a light intensity.

As shown in FIGS. 8B to 8E, rectangular signals (peaks) appear at thecentral area at the four scan lines of 121-th to 124-th. This signals(peaks) corresponds to the image formed by edge scattering light. Minusrectangular signals (peaks) on both sides of the central rectangularsignal (peak) correspond to horizontal sync signals of the video signal.

The signal waveforms like that of the third image were obtained also forthe upper fourth and fifth images shown in FIG. 7G. Rectangular signals(peaks) at the five scan lines were detected for the fourth image andrectangular peaks at the four scan lines were detected for the fifthimage. For the three clear images shown in FIG. 7G, therefore, clearrectangular signals were detected by the total of thirteen scan lines.

If scan lines in the odd field are considered, clear rectangularwaveforms are detected by the total of twenty six scan lines. The pitchof scan lines is 15 μm, and the optical magnification is 100. Therefore,detecting the rectangular signal (peak) waveform at each scan line meansdetecting a mark being disposed in the range of 26 [lines] ×15[μm/line]/100=3.9[μm] on the object surface along the X direction inFIG. 7A. This range size is generally the same as the detectable rangeby a conventional chromatic bifocal method. In order to enlarge thedetectable range, the pitch along the X direction between rectangularpatterns shown in FIG. 7A is made narrower.

If a signal waveform with a clear rectangular signal (peak) is obtained,the position can be detected through similar pattern matching.

With the above method, a position is detected by using light scatteredfrom only one set of edges of the wafer and mask marks. If the edgeshape of each mark changes because of variations of a mask formingprocess or a wafer manufacturing process, the position detectionprecision lowers. Next, a third embodiment will be described in whichthe number of edge patterns is increased to prevent the positiondetection precision from being lowered.

FIG. 9A is a plan view of alignment marks of the third embodiment of theinvention. Consider the coordinate system having the wafer surface as anX-Y plane and its normal direction as a Z-axis. A pair of wafer marks52A and 52B is disposed along the Y-axis direction, and a mask mark 62is interposed between the wafer marks 52A and 52B. Other embodiments tofollow will be described with the same coordinate system.

Each of the wafer marks 52A and 52B has the structure that mask patternslike that shown in FIG. 7A are disposed in three columns in the Y-axisdirection. Namely, rectangular patterns (edge patterns) 51 having edgesfor scattering incident light are disposed in a matrix shape along theX- and Y-axes. In FIG. 9A, three edge patterns 51 are disposed along theY-axis and five edge patterns 51 are disposed along the X-axis.Similarly, the mask mark 62 is constituted by edge patterns 61 disposedin a matrix shape.

FIG. 9B is a cross sectional view taken along one-dot-chain line B9--B9in FIG. 9A. Edge patterns 51 are formed on the surface of a wafer 50.Edge patterns 61 are formed on the bottom surface of a mask 60.

FIG. 9C is a cross sectional view taken along one-dot-chain line C9--C9in FIG. 9A. In each alignment mark, edge patterns 51 or 61 having alength W in the Y-axis direction are disposed along the Y-axis at apitch P. The distance between the centers of the wafer mark 52A and maskmark 62 is represented by y5, and the distance between the centers ofthe wafer mark 52B and mask mark 62 is represented by y6.

FIG. 9D1 shows an image signal which is obtained through observation ofedge scattering light from the wafer marks 52A and 52B shown in FIGS. 9Ato 9C along the oblique optical axis containing the X-Z plane. FIG. 9D2shows an image signal which is obtained through observation of edgescattering light from the mask mark 62 with the same optical system.FIG. 9D3 shows a composite image signal composed of two image signalsshown in FIG. 9D1 and 9D2. The abscissa represents a position along theY-axis direction, and the ordinate represents a signal intensity. Ineach alignment mark, three edge patterns disposed in the Y-axisdirection are on the flat plane perpendicular to the oblique opticalaxis. Therefore, the three edge patterns disposed in the Y-axisdirection can be aligned on the object surface of the observationoptical system, and edge scattering light from each edge pattern canform a clear image. Three rectangular signals (peaks) appear at each ofthe positions corresponding to the wafer marks 52A and 52B and mask mark62. The width of the rectangular signal (peak) is equal to the length Wof the edge pattern in the Y-axis direction, and the pitch betweenrectangular signals (peaks) is equal to the pitch P between edgepatterns disposed in the Y-axis direction.

FIG. 9E shows correlation values between the differential waveforms ofthe wafer mark 52A and mask mark 62 obtained from a differential imagesignal of the image signal shown in FIG. 9D3. The abscissa represents ashift amount Δy in the y-axis direction and the ordinate represents acorrelation value. In FIG. 9D3, the differential waveform of the wafermark 52A is moved parallel in the positive y-axis direction. When theright rectangular signal (peak) of the wafer mark 52A is superposed onthe left rectangular signal (peak) of the mask mark 62, the correlationvalue becomes large, and a peak a1 shown in FIG. 9E appears.

As the differential waveform is further moved in the positive y-axisdirection by the pitch P, the right and middle two rectangular signals(peaks) of the wafer mark 52A ere superposed respectively on the middleand left two rectangular signals (peaks) of the mask mark 62. Since thetwo pairs of rectangular signals (peaks) are superposed, the correlationvalue becomes larger than when one pair of rectangular signals (peaks)is superposed, and a rectangular signal (peak) a2 higher than therectangular signal (peak) a1 appears.

As the differential waveform is further moved in the positive y-axisdirection by the pitch P, the three rectangular signals (peaks) of thewafer mark 52A are superposed respectively on the three rectangularsignals (peaks) of the mask mark 62. The correlation value becomesmaximum and the highest peak a3 appears. As the waveform is furthermoved, peaks having generally the same heights as the peaks a2 and a1appear sequentially. The shift amount Δy giving the highest peak a3corresponds to the inter-center distance y5 between the wafer and maskmarks 52A and 62. The inter-center distance y6 between the wafer andmask marks 52B and 62 can be obtained in the similar manner.

As above, three edge patterns disposed in the Y-axis direction allow todetect edge scattering light from the three edge patterns at the sametime. Therefore, even if the shape of one edge pattern is deformed froman ideal shape because of variations of manufacture processes or thelike, the observations of edge scattering light from the other edgepatterns not deformed allows a high precision position detection. Thenumber of edge patterns disposed in the Y-axis direction is not limitedto three, but two or more edge patterns are expected to provide similareffects as above.

With the use of alignment marks shown in FIGS. 9A and 9C, slightly lowerpeaks a2 appear on both sides of the maximum peak a3 as shown in FIG.9E. If the peak a2 is erroneously judged to be the maximum peak, then acorrect position detection is impossible. This misjudgment becomeslikely to occur when the number of edge patterns disposed in the Y-axisdirection is increased or when the S/N ratio of the image signal lowers.

A fourth embodiment will be described in which alignment marks whichsuppress peak misjudgment are used. FIG. 10A is a cross sectional viewof alignment marks of the fourth embodiment. The plan layout of thealignment marks is the same as the third embodiment shown in FIG. 9A.Each of the alignment marks 52A, 52B and 62 has three edge patternsdisposed in the Y-axis direction. In each alignment mark, the edgelengths of the edge patterns along the Y-axis direction are not uniform.Each edge pattern is formed so that when one alignment mark is movedparallel in the Y-axis direction and superposed on another alignmentmark, the lengths of corresponding edges of the edge patterns becomecoincide with each other.

The edge length of the middle edge pattern of each alignment mark shownin FIG. 10A is W2, and the edge lengths of edge patterns on both side ofthe middle edge pattern are W1. In the example shown in FIG. 10A, W1>W2.In each alignment mark, the pitch of edge patterns in the Y-axisdirection is P. The inter-center distance between the wafer and maskmarks 52A and 62 is y5, and the inter-center distance between the waferand mask marks 52B and 62 is y6.

FIG. 10B shows an image signal obtained through observations of thealignment marks shown in FIG. 10A along the oblique axis in the X-Zplane. Three rectangular signals (peaks) are detected at positionscorresponding to respective wafer marks 52A and 52B and the mask mark62. In each alignment mark, the width of the middle rectangular signal(peak) is W2 and the widths of the rectangular signals (peaks) on bothsides of the middle rectangular signal (peak) are W1. In each alignmentmark, the pitch between rectangular signals (peaks) is the same as thepitch P between edge patterns in the Y-axis direction.

FIG. 10C shows a correlation value between differential waveforms of thewafer and mask marks 52A and 62 of a differential image signal of theimage signal shown in FIG. 10B. Similar to FIG. 9E, five rectangularsignals (peaks) appear. A rectangular signal (peak) b1 corresponds towhen the right rectangular signal (peak) of the wafer mark 52A issuperposed on the left rectangular signal (peak) of the mask mark 62. Arectangular signal (peak) b2 corresponds to when the right and middlerectangular signals (peaks) of the wafer mark 52A are superposed on themiddle and left rectangular signals (peaks) of the mask mark 62. Arectangular signal (peak) b3 corresponds to when the three rectangularsignals (peaks) of the wafer mark 52A are superposed on the threerectangular signals (peaks) of the mask mark 62.

In the state where the rectangular signal (peak) b2 appears, thecorrelation value is smaller than the case of the same widths of allrectangular signals (peaks) because the widths of superposed rectangularsignals (peaks) are different (W1>W2). As a result, the height of therectangular signal (peak) b2 is lower than the height of the rectangularsignal (peak) a2 shown in FIG. 9E. Since the ratio of the height of thehighest rectangular signal (peak) b3 to the heights of the rectangularsignals (peaks) b2 on both sides of the highest rectangular signal(peak) becomes large, the highest rectangular signal (peak) becomes hardto be erroneously judged.

In the example shown in FIG. 10A, the edge lengths of the edge patternsof each alignment mark are made irregular.

Instead, the edge lengths may be made uniform and the pitches betweenedge patterns may be made irregular. Alternatively, both the edgelengths and pitches may be made irregular. In order to suppress thegeneration of misalignment, the irregular degree of the edge lengths orpitches is preferably set to +/- 10% or higher.

In FIGS. 9E and 10C, the image signal of one of the wafer mark and maskmark is moved parallel to superpose the image signal to the other imagesignal and detect the relative position. Other methods may be used fordetecting the relative position. For example, the image signals of thewafer mark and mask mark may be folded or turned back at a plurality ofpoints near the centers of the image signals. The fold point having ahighest correlation coefficient is used as the center of the mark. Inthis manner, the center positions of the wafer and mask marks can beobtained and the position of each mark can be detected. In this case,each mark is formed so that both the sides of the center becomesymmetrical.

Next, a fifth embodiment will be described with reference to FIGS. 11Ato 11E, 12, 13A to 13C, and 14A and 14B.

FIG. 11A is a perspective view of one edge pattern of a wafer mark.Illumination light is obliquely applied along the oblique optical axisin the X-Z plane, and light scattered from the edge extending in theY-axis direction is observed. In this case, the image formed byscattered light has an intensity distribution given by the equation (4).Therefore, as shown in FIG. 11B, a long line image is obtained incorrespondence with the line spread function of the lens.

As shown in FIG. 11C, the length of the edge is shortened in the Y-axisdirection. As the edge length becomes shorter than the lens resolution,the intensity distribution O(x, y) of reflected light given by theequation (1) may be substituted by δ(x, y). Therefore, the equation (1)can be transformed into: ##EQU2## where PSF(x, y) represents a pointspread function of the lens.

If illumination light has continuous spectra, it can be expressed by:

    I(x, y)=∫PSFλ(x-Δxλ, y-Δyλ)dλ(7)

where λ represents a wavelength of light, PSFλ represents a point spreadfunction at the wavelength λ, Δxλ represents a lateral shift amount of apoint image in the X-axis direction caused by the lens chromaticaberration at the wavelength λ, Δyλ represents a lateral shift amount ofa point image in the Y-axis direction caused by the lens chromaticaberration at the wavelength λ, and the integration is performed for thewhole wavelength range.

As above, as the edge length is made equal to or shorter than the lensresolution, a point image such as shown in FIG. 11D can be obtainedwhich is analogous to the point spread function of the lens.

FIG. 11E is a perspective view of an edge pattern wherein illuminationlight is scattered near from the apex at which three planes cross. Inthis specification, a pattern having an edge which scatters illuminationlight and a pattern having an apex which scatters illumination light arecollectively called an edge pattern.

The image formed by light scattered near from the apex shown in FIG. 11Ecan be also considered to be analogous to the point spread functiongiven by the equations (6) and (7). On an SiO₂ film on a silicon wafer,an aluminum wafer mark is formed which has a square plan shape with aside length of 40 μm and is 523 nm thick. A resist film is coated overthe wafer mark to a thickness of 1.8 μm. Scattered light from the apexof the wafer mark was observed and a point image such as shown in FIG.11D was able to observe. The angle between the wafer normal directionand the illumination and observation optical axes was set to 30 degrees.

FIG. 12 shows an image signal of a point image formed by scattered lightfrom the apex. A spike-shaped peak at the center corresponds to thepoint image formed by scattered light. As shown in FIG. 12, a sharp peakhaving very small waveform distortion was obtained.

FIGS. 13A to 13C are plan views of mask marks and wafer marks having anapex which scatters illumination light. A mask mark 62 is disposedbetween wafer marks 52A and 52B.

The alignment marks 52A, 52B, and 62 shown in FIG. 13A each areconstituted by edge patterns having a square plan shape disposed inthree rows at a pitch P in the X-axis direction, and in two columns inthe Y-axis direction. One apex of each edge pattern of a square planshape is directed in the positive X-axis direction, i.e., toward theobservation optical axis direction.

An edge pattern shown in FIG. 13B has an isosceles right triangle planshape, and its apex is directed in the positive X-axis direction. Anedge pattern shown in FIG. 13C has a chevron plan shape, and its apex isdirected in the positive X-axis direction. The layouts of edge patternsconstituting the alignment marks shown in FIGS. 13B and 13C are similarto the alignment marks shown in FIG. 13A.

Position alignment between a wafer and a mask can be made by a methodsimilar to the method described with FIGS. 9A to 9E, through observationof scattered light from the apexes of the edge patterns disposed asshown in FIGS. 13A to 13C. In the method described with FIGS. 9A to 9E,the image signal is differentiated to obtain a correlation value.However, in this embodiment, the image signal formed by scattered lightfrom the apex has already a sharp peak so that the image signal itselfmay be used for obtaining a correlation value, without differentiation.

The triangle plan shape such as shown in FIG. 13B can narrow the pitch Pin the X-axis direction, as compared to the square plan shape. Thechevron plan shape such as shown in FIG. 13C can narrow the pitch stillfurther.

The factors of generating position detection errors contained in a lineimage and a point image analogous to the line spread function and thepoint spread function are anticipated to be different. If an errorfactor has a nature that error components are accumulated when an imageformed by scattered light is integrated in the longitudinal direction(i.e. Y direction in FIG. 11A), this error factor may considerablyaffect the line image although it may not affect the point image.Conversely, if an error factor has a nature that error components arecancelled out when an image formed by scattered light is integrated inthe longitudinal direction, this error factor may considerably affectthe point image although it may not affect the line image. It istherefore considered that the position detection error can be reducedwhen a line image is used or when a point image is used.

It can be expected that the total position detection error can bereduced if both an edge which forms a line image and an edge or apexwhich forms a point image are used in an alignment mark.

FIG. 14A is a cross sectional view of alignment marks according to thefifth embodiment. Wafer marks 52A and 52B are formed on the surface of awafer 50, and a mask mark 62 is formed on the bottom surface of a mask60. Each alignment mark is constituted by five edge patterns disposedalong the Y-axis direction. Of the five edge patterns, the edge patternsat opposite ends have their edge lengths in the Y-axis direction shorterthan the lens resolution such as shown in FIG. 11C, or have apexes forscattering illumination light such as shown in FIG. 11E.

FIG. 14B shows an image signal obtained through observation of edgescattering light from the alignment marks shown in FIG. 14A along theoblique optical axis direction in the X-Z plane. Five peaks appear ateach of the positions corresponding to the wafer marks 52A and 52B andthe mask mark 62. Of the five peaks, the peaks at opposite ends arenarrow and can be considered analogous to the point spread function ofthe lens. This image signal is differentiated, and the differentialimage signal is used for similar pattern matching to detect the relativeposition. The position detection can be performed therefore by usingboth the point and line images.

In the first to fifth embodiments, the method of reducing a positiondetection error has been described by disposing edge patterns along thenormal direction of the incidence plane. Next, a method of detecting aposition without being influenced by a proximity gap between a wafer anda mask will be described in which edge patterns are disposed in parallelto the incidence plane.

FIG. 15A is a plan view of a wafer mark according to the sixthembodiment. Twenty one rectangular edge patterns 70 are disposed in theX-axis direction at a pitch of 4 μm. This edge pattern column isdisposed in three columns in the Y-axis direction.

FIG. 15B is a schematic cross sectional view of a wafer mark and anoptical system in which the wafer mark is observed along the opticalaxis direction with an incidence angle of 30 degrees in the X-Z plane.Edge patterns 70 are formed on the surface of the wafer 71. Illuminationlight is applied coaxially with the oblique optical axis 73 andscattered light from the edges of the edge patterns 70 are observed. Abroken line 72 indicates the object surface of the object lens of theobservation optical system.

When the wafer 71 is at the position u1, the fifth edge pattern from theleft in FIG. 15B is positioned at the object surface 72. Next, as thewafer 71 is moved parallel along the optical axis 73 to positions u2 andu3, the third edge pattern from the left and the leftmost edge patternare positioned at the object surface 72.

If the pitch of edge patterns 70 in the X-axis direction is 4 μm, theedge patterns come the object surface one after another as the wafer 71is moved along the optical axis by 2 μm at a time. Therefore, if thedepth of focus of the lens is 1 μm, one of edge patterns is always onthe object surface and a clear image can be obtained.

FIG. 15C illustrates a dependency of a detecting precision upon a waferposition, the edge pattern position being detected through observationof wafer marks shown in FIG. 15A by the method described with FIG. 15B.The abscissa represents a serial number of an edge pattern in focus, andthe ordinate represents a detected value in the unit of nm. The detectedvalue defined is a half of a difference between spaces between an edgepattern at the middle in the Y-axis direction and one and the other ofedge patterns on both sides of the first mentioned edge pattern.

The motion distance of the wafer along the optical axis from theobservation state of the first edge pattern to the observation state ofthe twenty first edge pattern, is 40 μm. As shown in FIG. 15C, even ifthe wafer is moved by 40 μm, the detected value is within the range from-17 nm to +25 nm.

Even if the wafer is moved along the optical axis direction, theposition of an edge pattern can be detected relatively precisely. Whenthe wafer is also moved along the normal direction to the surface of thewafer, it can be detected relatively precisely. The main factor ofvariation in detected values may be variation of the shapes of edgepatterns. Therefore, as described in the third embodiment, a moreprecise position detection may be expected if a plurality set of edgepatterns are disposed in the Y-axis direction and a plurality of edgepatterns are observed at the same time.

The features of this method envisaged from the above experiment resultswill be described with reference to FIG. 16.

FIG. 16 is a cross sectional view of wafer and mask marks with aplurality of edge patterns disposed in parallel to the incidence plane.A broken line 72 indicates the object surface of an object lens of theobservation optical system.

When a wafer 71 is at a position v1 or v2 shown in FIG. 16, one of theedge patterns is on the object surface 72. Therefore, even if the wafer71 is at any one of the positions v1 and v2, an edge image by scatteredlight from the wafer and mask marks can be detected clearly. Since themask mark has a plurality of edge patterns disposed in the X-axisdirection, the edge image by scattered light from the mask mark can bedetected clearly even if the Z-axis position of the mask is shifted. Ifthe pitch between edge patterns is selected so that one of edge patternsenters the range of the depth of focus of the lens, the edge image canbe detected clearly even if the edge is not just on the object surface.

Stable position detection can be performed therefore even if thepositions of the wafer and mask in the Z-axis direction shift within acertain range. The gap between the wafer and mask can be obtained by amethod similar to that described with FIGS. 2C and 2D.

In FIGS. 15 and 16, observation of scattered light from a straight lineedge has been described. Also for the observation of scattered lightfrom an apex, stable position detection is ensured by disposing aplurality of edge patterns in the X-axis direction at a predeterminedpitch, even if the wafer and mask displace in the Z-axis direction. Thegap between the wafer and mask can also be obtained.

In the first to sixth embodiments, position alignment in the edgelongitudinal direction can be performed through oblique observation ofwafer and mask marks. Since the optical system is not required to beplaced in the exposure area, the position detection is possible evenduring exposure after the position alignment. Since the illumination andobservation optical axes are coaxial, there is no axis misalignment sothat an image always stable can be obtained.

Since telecentric illumination is used, a change in an image by edgescattering light can be suppressed if the edge displaces in the range ofthe depth of focus.

Regular reflection light of illumination light does not enter theobservation optical system, but only scattered light enters. Therefore,by adjusting the intensity of illumination light, an S/N ratio of animage formed by scattered light can be adjusted. Since regularreflection light does not enter, an underlying layer on a wafer does notadversely affect position detection. Edge scattering light utilizes ascattering phenomenon on uneven surfaces. Therefore, influence by lightinterference phenomenon and the like in a resist film can be eliminatedso that stable position detection is possible. Since incoherentillumination light can be used, there is no light interference at thegap between a wafer surface and a mask surface. Edge scattering lightcan be observed even if coherent light is used.

In the above embodiments, the angle between the observation optical axisand the normal to the exposure surface is set to 30 degrees. A clearimage by scattered light was obtained at the angle in the range from 15to 45 degrees.

In the above embodiments, the illumination and observation optical axesare set coaxially. The positional relationship between the illuminationand observation optical axes are not necessarily required to be coaxial,if as described earlier the regular reflection light of illuminationlight does not enter the observation optical system. For example, theillumination and observation optical axes may be set so that an anglebetween two line images of the axes vertically projected on the exposuresurface is smaller than 90 degrees.

FIG. 17A is a schematic plan view showing the positional relationshipbetween position aligning marks, an illumination optical system, and anobservation optical system wherein the illumination and observationoptical axes are set so that an angle between two line images of theaxes vertically projected on the exposure surface becomes smaller than90 degrees. Disposed in an exposure area EA are an X-axis positionaligning mark M_(X) and Y-axis position aligning marks M_(Y1) andM_(Y2). In FIG. 17A, the wafer and mask marks are shown as one mark.

With these three marks M_(X), M_(Y1), and M_(Y2), position alignment inthe X- and Y-axis directions and in the rotation direction (θ_(Z)direction) in the X-Y plane is possible.

Illumination light is applied from an illumination optical system L_(X)to the mark M_(X) and edge scattering light from the mark M_(X) isobserved by an observation optical system D_(X). Since an angle α_(X)between two line images of the illumination and observation optical axesvertically projected on the exposure surface is smaller than 90 degrees,both the optical systems D_(X) and L_(X) can be set on one side of theexposure area EA.

The illumination optical systems L_(Y1) and L_(Y2) and observationoptical systems D_(Y1) and D_(Y2) of the marks M_(Y1) and M_(Y2) canalso be set on one side of the exposure area EA.

The images of the illumination optical axis and observation optical axisvertically projected on the exposure surface may superpose one upon theother and only the angles between both the axes and the Z-axis are setdifferent.

FIG. 17B shows another arrangement in which the optical axes of theillumination optical systems L_(X), L_(Y1), and L_(Y2) shown in FIG. 17Aare set coaxial with the optical axes of the observation optical systemsD_(X), D_(Y1), and D_(Y2) by using half mirrors HM_(X), HM_(Y1), andHM_(Y2). The coaxial arrangement of the optical axes of the illuminationand observation optical systems facilitates the setting of opticalsystems.

FIG. 17C is a schematic plan view showing the positional relationshipbetween position aligning marks, an illumination optical system, and anobservation optical system wherein as in conventional positionalignment, illumination light is applied obliquely to the exposuresurface, and the image of a mark is observed by using regular reflectionlight from the mark. Similar to FIG. 17A, disposed in an exposure areaEA are an X-axis position aligning mark My and Y-axis position aligningmarks M_(Y1) and M_(Y2).

In order to observe light regularly reflected from a mark, theillumination and observation optical axes are required to be generallysymmetrical with a normal to the exposure surface. For example,illumination light is applied to a mark M_(X) downward from anillumination optical system L_(X), and regular reflection light isobserved by a detection optical system at a lower position in FIG. 17C.It is therefore necessary to dispose the illumination optical system andobservation optical system so as to face each other through the exposurearea. In order to prevent a shift of the relative position between theillumination and observation optical axes, it is preferable to mount theillumination and observation optical systems L_(X) and D_(X) on onefixing unit F_(X).

The illumination optical systems L_(Y1) and L_(Y2) and observationoptical systems D_(Y1) and D_(Y2) of the marks M_(Y1) and M_(Y2) arealso required to face one another through the exposure area. It is alsopreferable to mount the illumination and observation optical systemsL_(Y1) and D_(Y1) on one fixing unit F_(Y1) and to mount theillumination and observation optical systems L_(Y2) and D_(Y2) on onefixing unit F_(Y2). The setting of the optical systems around theexposure area EA becomes therefore complicated and the whole systembecomes bulky.

In contrast, the position aligning systems shown in FIGS. 17A and 17Bcan dispose the illumination and observation optical systems on one sideof the exposure area EA, simplifying the setting of the optical systems.It is therefore possible to make the whole system compact and facilitatethe optical axis adjustment. If the optical axes of the illumination andobservation optical systems are made coaxial as shown in FIG. 17B, theoptical axis adjustment is not necessary.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It is apparent to those skilled in the art that variousmodifications, improvements, combinations and the like can be madewithout departing from the scope of the appended claims.

I claim:
 1. A semiconductor substrate comprising:an exposure planeformed with a position aligning wafer mark having a plurality of edgetype or point type scattering sources for scattering incident light,said scattering sources being disposed in a direction perpendicular to aplane of incidence of said incident light, wherein said scatteringsources include at least two or more edge type scattering sources whoselengths are not uniform.
 2. A semiconductor substrate according to claim1, wherein at least three scattering sources are disposed in thedirection perpendicular to the plane of incidence of said incidencelight, the lengths of said scattering sources being not uniform.
 3. Asemiconductor substrate according to claim 1, wherein a plurality ofscattering sources are disposed in the direction parallel to the planeof incidence of said incident light.
 4. A semiconductor substratecomprising:an exposure plane formed with a position aligning wafer markhaving a plurality of edge type or point type scattering sources forscattering incident light, said scattering sources being disposed in adirection perpendicular to a plane of incidence of said incident light,wherein at least three scattering sources are disposed in a directionperpendicular to the plane of incidence of said incidence light, thelengths of said scattering sources being non-uniform.
 5. A semiconductorsubstrate according to claim 4, wherein a plurality of scatteringsources are disposed in the direction parallel to the plane of incidenceof said incident light.